Dynamic differential drive

ABSTRACT

A differential drive for rotation of a central shaft correspoding in contrast to a conventional axially displaced drawbar. The invention employs a harmonic drive gear arrangement having a stationary axle and a servo motor driven rotary axle to produce zero differential out of the radial spline when the servo driven axle is stationary. Servo controllled displacement occurs through rotation of the servo driven axle with a substantial reduction with respect to rotation of the stationary axle, e.g., in the order of 97:1.

CROSS-REFERENCE

Copending application, Ser. No. 07/095,071 filed on Sept. 3, 1987,issued 5/1/89 having common inventorship and assignee with the presentinvention.

BACKGROUND OF THE INVENTION

The use of reciprocal drawbars in NC Systems to produce displacement ofa cutting tool are known in the art.

An example is disclosed in U.S. Pat. No. 4,040,315 for MACHININGCROSS-FEED HEAD COUNTERWEIGHT SYSTEM wherein a reciprocal drawbaractuated by hydraulic cylinder or otherwise, coaxially with the centerof rotation, produces mechanical displacement of a cross-slide on therotatable tool body. High maintenance requirements for actuatingcylinders has created a need for an alternative servo drive. Currentservo systems rotate a ball screw which actuates a ball nut coupled tothe drawbar which creates axial movement.

BRIEF DESCRIPTION OF THE PRESENT INVENTION

Accordingly, the present invention is directed to employ differentialhead rotation for the displacement of a cutting tool. A servo motor iscoupled to a rotary harmonic drive which, in addition to acting as adifferential, operates as a speed reducer for a rotary shaft passingthrough the main drive spindle. The harmonic drive creates adifferential speed of rotation between the spindle and the rotary shaftwhich transmits feed to a tool head.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevation illustrating the generalarrangement of components of a preferred embodiment;

FIG. 2 is an enlarged fragmentary side elevation illustrating therelation of the principle components;

FIG. 3 is an enlarged side elevation of the harmonic drive unit per sefor differentially rotating the generating shaft.

FIG. 4 is an enlarged cross-sectional view of the harmonic drive.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

With reference to FIG. 1, the principle components of the present systeminclude tool head 10, spindle 11, drive motor 12, drive pulley with belt13, harmonic drive unit 14 and servo motor with drive mounting 15.

With reference to FIG. 2, rotating elements driven by pulley 16 includeharmonic drive unit housing 17, hollow spindle 18 and tool head 10.

Referring to FIG. 3, rotary shaft 34, forms an integral extension ofdifferential output element 40 of harmonic drive 36, mounted throughbearings 37 within housing 17. Output flange 38 is bolted at 39 tocircular spline ring 40 adjacent dynamic spline ring 41 in turn adjacentto circular spline ring 42 fixed within housing 17.

Tubular axle 43 is mounted through flange 44 to stationary bracket 45(FIG. 2), while internal axle 46 is driven by servo motor 15 to effectdifferential rotation of shaft 34 as required to effect displacement ofthe tool head 10.

One example of the Harmonic Drive is generally available from theHarmonic Drive Division of USM Corporation, a subsidiary of Emhart andmay be understood with reference to FIG. 4 illustrating the principle ofoperation.

Three concentric components include eliptical wave generator 47operating through anti-friction bearings 48 to deflect flexspline 49having teeth at the elliptical extremities engaging meshing teeth inrigid circular spline 50 with progressive engagement produced byrelative rotation of elliptical wave generator 47. As explained in anEmhart Harmonic Drive publication:

"The use of nonrigid body mechanics allows a continuous ellipticaldeflection wave to be induced in a nonrigid external gear, therebyproviding a continuous rolling mesh with a rigid, internal gear.

Since the teeth on the nonrigid Flexspline and the rigid Circular Splineare in continuous engagement and since the Flexspline has two teethfewer than the Circular Spline, one revolution of the input causesrelative motion between the Flexspline and the Circular Spline equal totwo teeth. Thus, with the Circular Spline rotationally fixed, theFlexspline will rotate in the opposite direction to the input at areduction ratio equal to the number of teeth on the Flexspline dividedby two.

The relative rotation may be seen by examining the motion of a singleFlexspline tooth over one-half an input revolution.

The tooth is fully engaged when the major axis of the Wave Generatorinput is at zero degrees. When the Wave Generator's major axis rotatesto 90 degrees, the tooth is disengaged. Full reengagement occurs in theadjacent Circular Spline tooth space when the major axi is rotated to180 degrees. This motion repeats as the major axis rotates another 180degrees back to zero, thereby producing the two tooth advancement perinput revolution.

The tooth is fully engaged when the major axis of the Wave Generatorinput is at zero degrees. When the Wave Generator's major axis rotatesto 90 degrees, the tooth is disengaged. Full reengagement occurs in theadjacent Circular Spline tooth space when the major axis is rotated to180 degrees. This motion repeats as the major axis rotates another 180degrees back to zero, thereby producing the two tooth advancement perinput revolution.

All tabulated Harmonic Drive gear reduction ratios assume the Flexsplineis the output member with the Circular Spline rotationally fixed.However, any of the drive elements may function as the input, output orfixed member depending on whether the gearing is used for speedreduction, speed increasing or differential operation."

Referring to FIG. 3, three rigid rings 40, 41 and 42 are employed eachwith a circular spline constructed as an internal gear. A pair of wavegenerators 47 are employed as seen at 54, and 56, each as an ellipticalball bearing assembly having a flexspline 49 constructed aS a nonrigidexternal gear engaging respectively both the center dynamic spline 41and one of the outer splines 40, 42. One of said wave generators 47 iskeyed to fixed axle tube 43 as shown at 51 and the other is keyed to theservo motor driven axle 46 as shown at 52. In the typical embodimentillustrated in FIG. 3, each of the outer circular splines is providedwith 194 teeth, while each of the flexsplines as well as the centerdynamic spline is provided with 192 teeth.

The operation of the harmonic drive gearing when both axles 43 and 46are stationary, as with the servo motor in brake mode, can be understoodby considering the effect of a single revolution of the spindle andhousing 17 producing a single revolution of the circular spline 42relative to the stationary wave generator held by key 51. The passage of194 teeth will advance the flexspline two teeth past a single revolutionwhich in turn will advance the central dynamic spline as well as theother flexspline each having an equal number of 192 teeth, two teethpast a single revolution, just sufficient to advance output circularspline 40 one revolution, thereby establishing a 1:1 synchronous speedof spindle 18 and shaft 34 corresponding to zero feed to tool head 10.

The effect of servo motor rotation of axle 46 may be understood byconsidering the effect of such rotation with the spindle and housing 17stationary holding circular spline 42 and central dynamic spline 41likewise stationary. In such case, rotation of the output wave generatorby key 52 one revolution actuating its flexspline with 192 teeth againstthe stationary dynamic spline having an equal number of teeth willproduce zero rotation of the flexspline but a relative two toothrotation of the output circular spline due to the 194 to 192 toothdifferential. Accordingly, any rotational speed of the servo motordriven axle 46 will produce a differential speed of shaft 34 relative tospindle speed equal to servo motor rpm divided by 97.

It will be noted that the 192 to 194 tooth differential between thecentral dynamic spline and the respective outer splines is necessary toachieve the dual purpose of zero feed when the servo motor is in a brakemode as well as a positive feed from any rotation of the servo motordriven axle 46. Thus, if the central dynamic spline were provided with194 teeth matching the outer dynamic splines, the first objective ofzero feed through synchronous drive shaft 34 would be obtained but thesecond objective of positive feed from rotation of axle 46 would bedefeated. This can be understood by considering the spindle and housing17 to be stationary, locking circular spline 42 and central spline 41against rotation in which case any rotation of axle 46 and associatedwave generator actuation of flexspline engaging central spline 41 andouter spline 40 would produce no relative movement of outer spline 40.

While the construction of flexsplines having a slightly smallercircumferential dimension to accommodate progressive tooth engagementthrough the wave generator involves a slightly greater pitch for thecentral dynamic spline, the division of the respective pitch circledifferential over 192 teeth renders the individual tooth pitchdifferential entirely tolerable without significant lost motion orbacklash.

I claim:
 1. Rotary tool comprising a rotary tool head, means fordisplacing a tool, rotary motor driven spindle with drive connection tosaid head, rotary shaft within said spindle, means responsive todifferential speeds of said respective spindle and shaft for producingdisplacement of said tool, and harmonic drive means for producing saiddifferential shaft and spindle speeds, said harmonic drive meansincludes a housing rotating at spindle speed, a fixed axle andconcentric rotational axle, a first rigid internally splined ring fixedto rotate with said housing, a second rigid internally spliced circularring connected to rotate with said shaft, an intermediate rigidinternally splined circular ring, a first eliptical wave generatorcoupled to said fixed axle having a flexible external spline mountedthereon with anti-friction bearings engaging internal splines of saidfirst and intermediate rings at diametric extremities of said ellipticalwave generator, a second elliptical wave generator fixed to saidrotational axle having a flexible spline mounted thereon withanti-friction bearings engaging the internal splines of said second andintermediate rings at diametric extremities of said second ellipticalwave generator, said drive means producing synchronous shaft and spindlespeeds when said rotational axle is stationary and differential speedsresponsive to rotation of said rotational axle.
 2. The combination ofclaim 1 including mechanical linkage means between said shaft and saidmeans for tool displacement responsive to relative rotation of saidshaft within said head.
 3. The combination of claim 1 wherein each ofsaid first and second circular spline rings is provided with two morespline teeth than said flexible splines and said intermediate splinering, said intermediate spline ring having slightly greater spline toothpitch than respective flexible splines to accommodate the differentialin respective pitch circumferences together with their respective equalnumber of teeth.
 4. The combination of claim 3 wherein the numbers ofspline teeth are respectively in the order of 194 and 192 providing arelative shaft speed differential from spindle speed in the order ofdriven axle speed divided by
 97. 5. The combination of claim 1 includinga common motor drive means for said spindle and harmonic drive meanshousing.
 6. The combination of claim 1 including a servo motor forbraking and driving said rotational axle.
 7. The combination of claim 6wherein said fixed axle comprises a hollow shaft with a fixed mountingkeyed to said first elliptical wave generator, and said rotational axleextends from said servo motor through said fixed axle to a keyedconnection with said second elliptical wave generator.
 8. Thecombination of claim 1 wherein said spline tooth engagement atelliptical extremities is accompanied by intermediate disengagement toaccommodate a two tooth differential for establishing a reduction ratioequal to two divided by the total number of teeth.